1. Field of the Invention
The present invention relates to a liquid delivery line purging or cleaning system where at least two lines are brought to a mixer for combining or mixing of the liquids and subsequent application of the combined/mixed liquids. One line is purged or cleaned with solvent and the other line with air. The method and system of the present invention add a small amount of solvent to the air in the air cleaning line which is used to purge or clean the other line while providing a constant air bleed and, if desired, a check valve.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn. 1.97-1.99.
Heretofore, in paint spray systems, where it is necessary to change the paint color available to the spray operator, a device called a color changer is employed. A color changer is a stack of valves, each valve supplied by an independent paint source such as a pump in a large container of a particular type of color of paint. Each valve supplies paint to a manifold common to all valves. By opening one valve of a particular color, the manifold then directs that paint to a conduit (hose) that supplies the paint applicator or application device, typically a paint spray gun, sometimes handheld, sometimes robotic, etc.
The type of applicator is irrelevant to the present invention. The purpose of the applicator is to start and stop the delivery of the paint and to atomize or otherwise dispense it. In applications where rapid changes from one color to another are necessary, color changers are employed.
This is the sequence of events during a color change using color changers:
1. The spray applicator is closed to stop fluid (paint) flow. PA1 2. The spray applicator is positioned to direct the waste paint into a waste container. PA1 3. The "opened" color valve is closed. PA1 4. The spray applicator is activated to relieve fluid pressure from the color changer through the paint supply hose and through an outlet from the spray applicator. PA1 5. An air color change valve (Air Purge Valve) on the color changer is opened, introducing compressed air to the color changer and paint hose to expel the waste paint through the hose and into the waste container. Regardless of the paint supply pressure, this air pressure typically is the maximum air supply pressure available in the plant, or typically never above approximately 120 psi. It is not unusual for this air supply pressure to be as low as 90 psi. PA1 6. Once the compressed air evacuates the paint hose, the air purge valve is closed and a solvent color change valve (Solvent Purge Valve) is opened. The purpose of the solvent is to wash away any old paint that still clings to the walls of the color changer manifold, hoses, and spray applicator. Solvent pressure is typically set to be at the same pressure as the paint pressures that supply the paint valves. There are typically three groups of paint supply pressures, depending on the spray applicator used in the paint process: PA1 7. From this point, the solvent and air purge valves alternately open and close to provide an alternating solvent and air scrub that is more effective in a cleaning action than by solvent itself. The cycle may continue until the operator stops it, or it may be automatically controlled by a computer or other electromechanical device. When the operator does not open or vent the spray applicator, problems can occur. If the spray applicator is not opened before the color change process begins, or if the spray applicator is closed during the purge process, the purge process will accomplish nothing since the waste does not have anywhere to go.
A. Airspray--50 to 120 psi PA2 B. Air Assisted Airless--700 to 1200 psi PA2 C. Airless--1200 to 3000 psi
There are many variations of these categories depending on paint viscosity, etc., and a wide range of paint pressures relative to the air pressure are available to the air purge valve.
The air purge valve will open to blow the paint hose out with 100 (plus or minus) psi. The paint hose, ballooned up from the paint pressure, and, given that the spray applicator is not activated to release this pressure, the old paint will move backwards through the air purge valve and into the compressed air supply lines. When the air pressure and the decreased fluid pressure balance, the backwards flow of paint will stop, and will then move forward again when the operator activates the spray applicator. This error is minimized by the use of check valves on all paint valves on the color changer, as well as on the solvent and air purge valves. There is, however, a small cavity in the air purge valve, between the valve seat and the face of the check valve. This cavity in the air purge valve holds compressed air which will be further compressed by the introduction into this cavity of the higher pressure paint when the air purge valve opens. The problem becomes more and more severe as paint pressures get higher. In fully automatic paint systems, such as robotic, where the spray gun trigger can be activated automatically prior to the color change sequencing, this is usually not an issue, though the robot has no way of knowing if the spray applicator dispensing tip is plugged, at which time there could still be a backflow issue because the pressurized paint has nowhere to go but backwards.
Once the air purge valve needle and seat are contaminated by paint, the air purge valve is subject to leaking. It will be a matter of time before the check valve becomes contaminated too. Any time after that, the compressed air supply may be subject to the same failure mode as described above. Note that the compressed air passing through the air purge valve and associated check valve does a poor job of cleaning, and is not intended to do any real cleaning at all, but only is intended to push old paint out of the way so that the cleaning solvent can do its job.
In addition, there are two component paints that, until both components are blended together (catalyzed), the components remain generally fluid. Once mixing is initiated, however, those paints will turn into a hard material over time. The only way to stop this hardening is to break down the material by flushing with solvent. Should this catalyzed material contaminate the air purge valve, it is a fact that the valve needle and seat will leak causing catalyzed paint to move into the compressed air supply.
It is possible to use solvent in all lines, "solvent only", to clean the paint passages, thereby eliminating the compressed air altogether. Some paint or liquid mixing systems actually do this. The chief disadvantage to doing this is that the solvent usage necessary to clean the system increases greatly, in volume, to accomplish the same level of cleaning as when using compressed air along with the solvent. However, due to solvent cost, waste solvent disposal costs, and Environmental Protection Agency limits on hazardous waste generation such as waste disposal and solvent emissions, this is not the desirable option.
The layout of the valving in some two component mixing systems is such that the solvent purge valve is located to clean the catalyst passageways, and the air purge valve is located to clean the resin passageways. Solvent and air come together in the same cavity that the resin and catalyst come together to mix. The reason that the solvent is installed on the catalyst side of the mixing system is that the catalyst is subject to degradation when exposed to air, or more specifically, the moisture (water, humidity) present in air. The catalyst side is, therefore, protected by the presence of solvent. In order to effect the most efficient cleaning procedure, the air, therefore, is introduced on the resin side. Protected by check valves, this layout is no different in operation than the paint color changing system described above.
Eventually the air purge valve needle and seat will become contaminated, be subject to leaking, and eventually be subject to allowing mixed paint and/or solvents to move backwards through the compressed air supply system.
It has been proposed to provide an air bleed hole in a pressure system to prevent backflow as described in the Carlton U.S. Pat. No. 2,389,413. Also it has been proposed in the Konieczynski U.S. Pat. No. 5,016,665 to provide a fluid bleed device in a liquid application system.
However, it has not been proposed heretofore to provide an air bleed backflow monitoring device in conjunction with an air and solvent mixing device.